Adar Pelah

Self-motion perception during locomotor recalibration: more than meets the eye.

Do locomotor aftereffects depend specifically on visual feedback? In 7 experiments, 116 college students were tested, with closed eyes, at stationary running or at walking to a previewed target after adaptation, with closed eyes, to treadmill locomotion. Subjects showed faster inadvertent drift during stationary running and increased distance (overshoot) when walking to a target. Overshoot seemed to saturate (i.e., reach a ceiling) at 17% after as little as 1 min of adaptation. Sidestepping at test reduced overshoot, suggesting motor specificity. But inadvertent drift effects were decreased if the eyes were open and the treadmill was drawn through the environment during adaptation, indicating that these effects involve self-motion perception. Differences in expression of inadvertent drift and of overshoot after adaptation to treadmill locomotion may have been due to different sets of ancillary cues available for the 2 tasks. Self-motion perception is multimodal.

Visuomotor adaptation without vision

Abstract In 1995, an aftereffect following treadmill running was described, in which people would inadvertently advance when attempting to run in place on solid ground with their eyes closed. Although originally induced from treadmill running, the running-in-place aftereffect is argued here to result from the absence of sensory information specifying advancement during running. In a series of experiments in which visual information was systematically manipulated, aftereffect strength (AE), measured as the proportional increase (post-test/pre-test) in forward drift while attempting to run in place with eyes closed, was found to be inversely related to the amount of geometrically correct optical flow provided during induction. In particular, experiment 1 (n=20) demonstrated that the same aftereffect was not limited to treadmill running, but could also be strongly generated by running behind a golf-cart when the eyes were closed (AE=1.93), but not when the eyes were open (AE=1.16). Conversely, experiment 2 (n=39) showed that simulating an expanding flow field, albeit crudely, during treadmill running was insufficient to eliminate the aftereffect. Reducing ambient auditory information by means of earplugs increased the total distances inadvertently advanced while attempting to run in one place by a factor of two, both before and after adaptation, but did not influence the ratio of change produced by adaptation. It is concluded that the running-in-place aftereffect may result from a recalibration of visuomotor control systems that takes place even in the absence of visual input.

Editorial: Walking in real and virtual environments

This special issue is devoted to understanding human movement by walking in real and virtual environments, investigating activities including obstacle avoidance, estimation of travel distance, visuomotor calibration of walking, gender differences in path integration, heading assessment in low vision and visual speed-matching on a feedback-controlled walking machine (treadmill). Whether implicitly or explicitly, every study in the issue takes a comparative approach, relating the activity in the virtual world to its counterpart in the real world, and vice versa, as without cross-validating one realm with the other it is arguably impossible to get it right. Necessarily, this is also a multidisciplinary area of research and thus, appropriately, the background of the contributors to this issue range from computer science to psychology, engineering to neuroscience, optics to ophthalmology and others. Indeed, there is as much of a goal for the issue to understand multimodal processing in human locomotion as there is to further the technology of virtual reality through its understanding, two aspects that embody the cross-disciplinary ethos of ACM TAP. The capability for a user to move around in the virtual, as in the real, world has long been a key requirement and challenge for virtual reality technologists and scientists. Full motion for navigation in virtual environments (VEs) using joy-stick, mouse or keyboard, can be effective but does not mimic important aspects of human action nor, it seems, do all graphic environments convey navigational cues equivalently. What can we learn from human locomotor movement and behavior in the real world that could yield a better interface in virtual reality (VR), and how can we use VR to improve our understanding of the former? Obstacle avoidance is no less than a requirement to our survival while moving, and thus to faithfully include it in VR requires a comparison to be made between its characteristics in VR and the real world. In Fink et al. in this issue, small but reliable differences are found in locomotor paths, as subjects’ avoided

Matching visual and nonvisual signals: evidence for a mechanism to discount optic flow during locomotion

We report on recent experiments to investigate the Arthrovisual Locomotor Effect (ALE), a mechanism based on non-visual signals postulated to discount or remove the self-generated visual motion signals during locomotion. It is shown that perceptual matches made by standing subjects to a constant motion optic flow stimulus that is viewed while walking on a treadmill are linearly reduced by walking speed, a measure of the reported ALE. The degree of reduction in perceived speed depends on the similarity of the motor activity to natural locomotion, thus for the four activities tested, ALE strength is ranked as follows: Walking > Cycling > Hand Pedalling > Finger Tapping = 0. Other variations and important controls for the ALE are described.

The coupling of vision with locomotion in cortical blindness

Maintaining or modifying the speed and direction of locomotion requires the coupling of the locomotion with the retinal optic flow that it generates. It is shown that this essential behavioral capability, which requires on-line neural control, is preserved in the cortically blind hemifield of a hemianope. In experiments, optic flow stimuli were presented to either the normal or blind hemifield while the patient was walking on a treadmill. Little difference was found between the hemifields with respect to the coupling (i.e. co-dependency) of optic flow detection with locomotion. Even in the cortically blind hemifield, faster walking resulted in the perceptual slowing of detected optic flow, and self-selected locomotion speeds demonstrated behavioral discrimination between different optic flow speeds. The results indicate that the processing of optic flow, and thereby on-line visuo-locomotor coupling, can take place along neural pathways that function without processing in Area V1, and thus in the absence of conscious intervention. These and earlier findings suggest that optic flow and object motion are processed in parallel along with correlated non-visual locomotion signals. Extrastriate interactions may be responsible for discounting the optical effects of locomotion on the perceived direction of object motion, and maintaining visually guided self-motion.

The perceptual influences on gait transition of step parameters and optic flow in virtual environment locomotion simulators

Simulation of natural locomotion in virtual environments requires an understanding of self-motion perception derived from both gait and sensory parameters. The walk-run transition (WRT) is a clearly-defined physical gait descriptor that emerges largely from a users assessment of their own speed. The WRT depends not only on the biomechanics, kinetics and energetics of gait, but also on perceptual and cognitive factors, such as optic flow [Mohler et al. 2004; 2007], attention [Pelah et al 2006] and step frequency [Durgin et al 2007]. To further investigate gait transitions and the mechanisms for perception of self motion speed, 15 subjects were required to advance at prescribed step frequencies with increasing speeds on a treadmill, with or without optic flow, in a large-screen virtual environment. WRT speed reached its highest value at approximately 145 steps/min (0.61 Fr) for all conditions, decreasing at higher step frequencies. Above this frequency, optic flow reduced WRT speeds as step frequency increased, suggesting an enhancement of visuomotor interactions in locomotion simulators due to step frequency being forced beyond the natural step frequency range.

Perceptual rules for watermarking images: a psychophysical study of the visual basis for digital pattern encryption

We have measured the contrast detection thresholds for small bandpass targets embedded in digitized monochrome photographs of natural scenes. The targets are used to probe the properties of watermarking patterns, which might be embedded in a photography to state copyright or authenticity, while remaining invisible to a human observer. Thresholds were measured for targets embedded in different parts of the photographs in order to determine where in a photographs it would be most suitable to hide a watermarking pattern. Thresholds were also compared when the photographs were bandpass filtered or notch filtered in order to determine how the localized spectral energy in the photograph affected the visibility of a potential watermarking pattern. We also studied the visibility of targets embedded in synthetic pictures, whose spectral amplitude was similar to that of natural scenes. The test targets were most easily visible when embedded in parts of photographs where the luminance was relatively uniform, and they were especially easy to see where the average luminance was low. This was explicable on a simple model of contrast encoding in the human visual system. The targets were much harder to see when embedded in contrast-rich parts of the digitized photographs. Indeed, the thresholds were evaluated more than the simple human model predicted: the spatially- localized contrast energy in the photograph masked the test target effectively. The experiments with notch-filtered photographs produced surprising results that were not predicted at all by the human model. Even when the spectral energy was removed from the photograph in the band occupied by the test target, there was still substantial masking. This implies considerable masking between visual primitives encoding different spectral bands. It also implies that watermarking technology might be facilitated, since any contrast energy may hide a watermarking target regardless of their respective spectral content.

Visual stress symptoms from stereoscopic television

Visual stress from watching 3D TV and other stereoscopic displays has been reported by a number of studies. Alarming media reports of concerns for public health prompted the 3D industry to issue consumer guidance and warnings emphasising best practice and caution for susceptible groups. This study considers the importance of the problem by addressing the questions of prevalence, magnitude and significance of a range of symptoms and preferences associated with 3D viewing. A large cohort of 52 subjects (likely the largest used in such studies) with no pre-existing optometric or medical conditions viewed randomised 10 minute sessions in 3D or 2D. Results revealed a wide range of adverse symptoms of high magnitude and statistical significance for 3D, whether measured as a comparison between 3D and 2D or independently for each condition. In addition, an online survey of 106 participants concurred with subject preferences in indicating that 48% of respondents do not find 3D TV comfortable to watch. We conclude that visual stress and discomfort associated with 3D viewing is prevalent for a significant proportion of the healthy consumer population, and propose that the 3D industry address the problem on public health and commercial grounds.

Influence of step frequency on visual speed perception during locomotion

[Thurrell et al. 1998] first observed that the perceived speed of optic flow decreases in linear proportion to the increasing physical speed of locomotion on a treadmill and proposed this as a mechanism to discount from the visual signal retinal motion due to self-motion, described as an arthrovisual effect [Thurrell and Pelah 2005]. Since human locomotion consists of a complex of articulated movement, step parameters and associated afferent, efferent and efference copy signals, questions arise as to the relative contributions of these component messages to the reduction in the perception of optic flow speed (POFS). Here we report experiments [Casey 2010] on the role of step frequency (SF) previously proposed as a reliable estimate for perception of the speed of self-motion [Durgin et al. 2007; Dong et al. 2008].

Visibility of simple features on complex backgrounds

An essential task of the human visual system is to detect the presence of objects embedded within spatial backgrounds of the kind found in common visual scenes. Object backgrounds can differ from regions of effectively uniform luminance to areas of almost arbitrarily complex spatial structure. Detection on uniform backgrounds classically shows Weber behavior, but the form taken by the threshold function in the general case of spatially variable backgrounds is not known. For this it is proposed that a general expression of Webers law would apply, that accounts for the local contrast contributions both of the background and the feature. The new general law states that threshold is reached when feature contrast exceeds background contrast by an amount equal to a typical Weber constant. To define contrast on a spatially variable background, an adaptable contrast metric is used with variable position and spatial scale terms. It is shown that the detection thresholds of transient (triangle-profile) pulses at any phase on a sinusoidal background follow the general but not the familiar expression of Webers law. The results also demonstrate that for highly localized features local contrast can be computed at scales as fine as 0.6 arcmin. Further implications including the possible neural locus of the implied adaptation to local contrast are discussed.